Even if we one day have rovers, balloons
and airplanes continuously moving around the surface of Mars, we
should never judge a planet by its cover. We suspect that today's
desert-like Martian surface covers an aquifer of liquid water at some
level deep underground. To pursue our goal of "following the water"
in search of signs of life, we must gather radar images of the top
layers of crust and eventually send subsurface explorers to look
for the water.
Although we do not know much about
the subsurface, data from our Viking Missions
and Mars Global Surveyor
have given us abundant evidence of ancient surface water, including
streams and probably ancient oceans. While there is photographic
evidence for recent gullies, possibly cut by flowing water, we can see
no evidence for liquid water currently at the Martian surface.
We draw some of our theories for a
subsurface presence of water by looking at Earth. Even in desert regions,
we know that water can be abundant in the subsurface. We also know
that Mars is colder than Earth. From this information, we can speculate
that the subsurface of Mars may resemble some of the colder parts of
Earth. For example, in Antarctica or Iceland, we know that water is
stored in a layer of permafrost and beneath that, as liquid groundwater.
Even if the ancient surface water on Mars evaporated, there may still be
substantial reservoirs of water, in either liquid or frozen form, in the
Studying the geophysics of the crust is the
first step in figuring out what the subsurface is like. The very first
subsurface exploration of Mars for NASA will be in partnership with the
European Space Agency (ESA). In 2003, ESA will be sending their Mars Express
spacecraft to the Red Planet. This spacecraft will be carrying a
subsurface sounding instrument that will use a 40-meter (130 feet)
antenna to attempt to detect and map subsurface water.
Electric signals will be sent down the
antenna, creating low-frequency radar waves. The radar waves will
penetrate the Martian surface as deep as five kilometers (three miles)
and will be reflected back to the spacecraft by different subsurface
features, including water. This will give us a three-dimensional
understanding of where and how much water may be distributed in
the Martian subsurface. Only with this knowledge can we go to the
next stage of exploration, which is drilling.
Subsurface Exploration Technology
If we identify a geophysical signature for water from orbit or surface
studies, the next step is to drill in that location. To get to the zone
where frozen water--and possible dormant life--might be present, we
expect to drill to a depth of 200 meters. Liquid groundwater will be
even deeper. That's no easy feat, but it's critical for understanding the
possibility of past or present life on Mars and for confirming that water
resources are available for future human explorers.
Deep subsurface access on Mars will
have unique challenges. First of all, unlike on Earth, we will not be
able to use a drill to go through mud, water or probably even gas pressure to carry
the cuttings away from the bit. We will need new systems for fluidless
drilling. Second, we will need an effective means of keeping the hole
open while the drilling is proceeding. On Earth, this is normally done
with steel casing, which is very heavy. We are actively seeking
alternative ways of doing this on Mars that don't require so much
mass. Finally, we will have to develop autonomous control systems
that allow the drill to make operational decisions for itself. On Earth
we know that drills can get stuck very quickly, so we will need to teach
a Mars robotic drill or subsurface explorer how to recognize, avoid and
solve problem situations.
One device that scientists are considering
for underground exploration is a robotic mole. Mimicking the behavior of
the small furry Earth-bound creatures that burrow into the ground, robotic
moles will drill underground by pulverizing rock and soil, avoiding the need
for a complex drill stem. The European Space Agency (ESA) plans to
carry the first robotic mole to Mars aboard the Beagle Lander deployed
from Mars Express launched in 2003
but this device will have the ability to penetrate less than a meter
below the surface. A much more capable mole under development
in NASA's technology program, weighing about 20 kilograms, will be
capable of drilling hundreds of meters into the ground and possibly
deeper at a rate of 10-20 meters a day. Excavated soil will be moved
to the back of the mole and a small tube leading to the surface will help
alleviate the pressure from the growing mounds of soil. The tube will
also be used to send soil samples back to the surface and carry power
to the robotic mole.
The moles will look for many things as
they drill into the Martian subsurface. Most importantly, they will search
for liquid water. If liquid water is found, it could lead to discovery of
extinct life or possibly even present-day life. Any indications of life would
not likely be bigger than microbes. Even if the moles are unable to
find liquid water, scientists hope they will better determine the depth
where liquid water might exist by calculating how quickly the ground
heats up as the mole goes deeper.
The samples sent up to the surface
would be studied for scientific data such as mineral content and
oxidation levels of soil in the subsurface. A mole drilling at the polar
cap would study the layers of ice which tell the story of its history,
much like the rings of a tree reveal many things from its past. All of this
data would be clues in the search for ancient, or possibly current, life.